Effect of beta-carotene on content of some metabolites of lipid peroxidation and activity of ornithine decarboxylase was studied in rat gastric mucosal membrane during gastric carcinogenesis developed after administration of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Intragastric administration of MNNG led to a significant increase in content of diene ketones and malonic dialdehyde in the gastric mucosal membrane within a day, while beta-carotene normalized or decreased considerably the MNNG produced accumulation of these lipid peroxidation products. In acute experiments with MNNG beta-carotene administered per os at a dose of 0.5 mg per animal did not affect the pronounced activation of ornithine decarboxylase in the gastric mucosal membrane. However, in chronic experiments with MNNG repeated administration of beta-carotene led to statistically significant decrease of the constitution-dependent enzymatic activation in the gastric mucosal membrane and to inhibition of locus formation with abnormally high activity of ornithine decarboxylase. The findings suggest that anticarcinogenic effect of the natural antioxidant beta-carotene was shown at the step of carcinogenesis promotion.